A dynamic random access memory (DRAM) stores data by accumulation of electric charge in a capacitor. Thus, the off-state current of a transistor for controlling the supply of electric charge to the capacitor is preferably small. This is because small off-state current can increase the data holding period, and the frequency of refresh operation can be reduced.
Transistors including an oxide semiconductor (preferably an oxide including In, Ga, and Zn) in their channel formation regions are known. It is known that a transistor including an oxide semiconductor (preferably an oxide including In, Ga, and Zn) in a channel formation region has an extremely small off-state current.
Patent Document 1 describes a semiconductor device which can hold stored content for a long time by using a transistor including a channel formation region in an oxide semiconductor film. Patent Document 1 also describes a semiconductor device in which the chip area can be reduced by stacking a driver circuit and a memory circuit. Patent Document 2 describes a semiconductor device in which capacitance of a capacitor can be reduced by a divided bit line system.